Polyolefins and polyolefin compositions are typically used in products that are subjected to elevated temperatures which accelerate the rate of oxidation degradation in the polyolefin composition, commonly referred to as "heat aging", where long term performance is desired or required. Thus, the ability of polyolefins and compositions thereof to retain their original properties, such as elongation and tensile strength, at elevated temperatures during the desired or required term of performance is important.
Various antioxidant compounds or combinations thereof have been found to have a retarding or stabilizing effect on the "heat aging" of polyolefins and polyolefin compositions, and therefore have been widely used. For example U.S. Pat. No. 2,997,456 discloses stabilization of polyolefins against thermal degradation by incorporating metal salts of mercapto, selenide or telluride benzimidazole compounds into the polyolefins.
U.S. Pat. No. 4,036,912 discloses thermoplastic blends of ethylene-propylene or ethylene-propylene-diene polymer having a high ethylene sequence index and polypropylene. The reference teaches that antioxidants can be added.
U.S. Pat. Nos. 4,260,661 and 4,824,883 discloses using an antioxidant system comprising a combination of a zinc salt of a mercaptoimidazole with a sterically hindered di-tertiary butylphenol in polyethylene containing polymers to obtain improved heat curing stability.
Japanese Patent 58-122943 discloses a synergistic combination of antioxidants comprising a phenol, a 1,2-dihydroquinoline and an organic zinc salt, e.g. zinc salts of mercaptobenzimidazole and mercaptobenzothiazole, to improve the heat stability of polyolefins.
While the polyolefin compositions of the above-mention prior art give improved heat aging properties by incorporating antioxidants or combinations thereof, the percent retention of these original properties is still low.